This invention relates to the method and apparatus for charging non-conductive powder particles. More particularly, the invention is directed to a method and apparatus for electrostatic spray coating wherein a charge is imparted to particles and thereafter the particles are transported to the surfaces of a workpiece which is to be coated with the particles. The charge on the particles holds them to the workpiece by electrostatic attraction.
In a powder coating system, articles to be coated are normally conveyed through a powder booth. There an electrostatic charge is imparted to the powder particles and the charged particles are delivered to the object to be coated. The object is maintained at ground potential so that the charged particles adhere to the object because of the force induced between the charged particles and the grounded object. A significant percentage of the particles delivered toward the workpiece do not adhere to it and drop to the bottom of the powder booth. Those particles are recirculated through the powder transport system for reuse.
The charging and delivery system for the powder has primarily been by the use of electrostatic guns. Each gun usually has a passageway of circular cross section through which powder, entrained in a stream of air, passes. Electrodes within the passageway create a corona discharge which ionizes the air surrounding the electrodes. The particles, entrained in the air, collide with the ions in the air and acquire these ionic charges.
The gun electrode is maintained at 30 kV to 100 kV DC with respect to the workpiece, which is normally maintained at ground potential. The high voltage between the gun electrode and the workpiece creates an electric field. The charged particles tend to follow the electric lines of force created by the electric field between the gun and the workpiece.
The foregoing system which has been generally described and which is in widespread use has several disadvantages.
One of the disadvantages is referred to as Faraday caging. The electric force lines between the gun and the workpiece do not penetrate well into recesses in the workpiece. As a consequence, the powder which tends to follow the electrical force lines does not penetrate well into the recesses resulting in uneven coating thickness throughout the workpiece.
Further, of the original ions created by the corona discharge, only about 0.1-0.5% of them are involved in collisions with the particles which cause the ions to give up their charge to the particles. The rest of the ions follow the force lines and travel to the workpiece. As the ions build up on the surface of the workpiece, they create a surface charge of the same polarity as the charged particles and thus tend to repel the oncoming particles.